The present invention relates to a structure of a ferrite core half, and in particular, relates to such core half for use in forming the cores of a transformer or a choke coil in a power supply circuit. The example of a ferrite core disclosed is intended to be used in a transformer or a choke coil in a power supply circuit capable of handling up to 1 KW.
When used as a power transformer, it may form part of a DC-AC converter and, in this case, a primary power supply is applied to the transformer through a switching circuit to apply an alternate current input to the transformer, and then the required secondary voltage is obtained at the output of the transformer.
A ferrite core for such purposes must satisfy the following conditions.
(a) The core must not magnetically saturate, and preferably, the cross section along the magnetic path is constant along the whole magnetic path in the core.
(b) The core is preferably closed to improve the shield effect so that it does not disturb an external circuit.
(c) The shape of a core is preferably simple and enables a bobbin containing a winding coil or coils to be mounted on it and enables lead wires of the windings to extend outside of the core.
(d) The core is preferably as small and as light in weight as possible. Also, the power handling capacity to weight ratio should be as large as possible.
(e) The height of the transformer composed by the core is as low as possible, so that the transformer may be mounted on a printed circuit board.
The most popular conventional ferrite core has an E-shape having a constant cross section throughout. Alternatively, a combination of an E-shaped and an I-shaped core is used. However, such a combination core has the disadvantages that it is large in size, its shielding effect is not perfect and further, a bobbin to fit over the core and carry the coil windings must be rectangular in cross-section. Thus the windings are bent sharply at the corners of the bobbin and the normal insulation is often damaged, and further, undesirable leakage inductance increases.
We have proposed the improved core in the U.S. Pat. No. 4,352,080, which intends to improve the above condition (e). That core is intended to mount on a printed circuit board so that the core axis is perpendicular to the printed circuit plane. However, it has the disadvantage that the height of the transformer using the core is still high, and therefore, the mounting density of the components of a printed circuit board can not be high.
We have proposed another ferrite core in EP 68745 B1 (U.S. Pat. No. 4,424,504), which is shown in FIGS. 4 and 5. In those figures, the ferrite core 1 has a substantially rectangular base plate 2 which has a recess 2A, a circular center core 3 mounted on the center of the base plate 2, and a pair of side legs 4 mounted on both sides of the base plate 2. The center core 3 is positioned so that it contacts both the recess 2A and one side of the base plate 2. The cross section of each side leg 4 has linear lines 4A, 4B, 4C, a curve 4D, an arc 4E which is parallel with the center core 3, and the linear line 4F between the arc 4E and the line 4C.
The bobbin 11 which fits the core has hollow cylindrical body 12 with a pair of flanges 12A at both the ends of the same, and the terminal plate 13 having a plurality of terminal pins 13A. The terminal plate 13 is composed integrally with the body 12 by using a plastic material. A transformer or a choke coil is obtained by winding a coil on a bobbin into which a pair of cores are inserted. The transformer is mounted on a printed circuit board so that the pins 13A pass through the printed circuit board. The structure of the core of FIGS. 4 and 5 has the advantage that the height of the transformer on a printed circuit board is lower than that of the former one (U.S. Pat. No. 4,352,080), because the core is mounted on a printed circuit board so that the axis of the circular center core 3 is parallel to the plane of the printed circuit board.
However, it still has the disadvantage that the height on a printed circuit board is not low enough for a miniaturized electronic component. When the spacing between printed circuit boards is restricted to 25.4 mm, the height of a transformer on the board must be less than 16 mm, considering the spacing necessary for insulation and soldering.
Another approach for lowering the height of a transformer is the use of a core with a flat center core which has the enough cross sectional area for magnetic flux. However, when the center core is rectangular having sharp corners, it still has the disadvantage that the coil wound on a core can not fit well with the core at the corners, because the coil does not bend at the corners, but curves. So, some spacing is left between a core and a coil, and that spacing increases undesirable leakage inductance. Further, when the corners of the core are sharp, the coil would be injured, and the insulation of the coil would be damaged or destroyed.